Paleomagnetism and Seafloor Spreading

Slides from lectures preceding Seaflor Spreading exercise

Eileen Herrstromherrstro@illinois.edu

2018

Alfred Wegener

§  German meteorologist•  Who?

•  What?§  All land was once one large continent§  Later, it broke apart, moved to present places

•  When?§  Early 1900s

§  Wegener had lots of data… •  Why?

https://commons.wikimedia.org/wiki/File:Alfred_Wegener_ca.1924-30.jpg

Continental Drift •  “Pangaea” means “all lands”

https://commons.wikimedia.org/wiki/File:Pangaea_to_present.gif

Evidence From Fossils

§  Could not swim or float across oceans

•  Found on different continents

• With Pangaea, they could stay on land!

§ How did they get there??

https://commons.wikimedia.org/wiki/File:Snider-Pellegrini_Wegener_fossil_map.svg

Evidence from Paleoclimates • Climate

interpreted from rocks:

• Paleoclimates make sense with Pangaea

http://geophile.net/Lessons/PlateTectonics/PlateTectonics_03.html

§ Limestone = tropical reefs

§ Rock salt = deserts

Reactions

• Wegener thought his evidence was convincing.

•  Others:§ “Utter, damned rot!”

- president of American Philosophical Society

§  It “befogs the minds of students” -geologist B. Willis

After Wegener •  Little work on drift for next 20 years

•  1950s: new data were obtained

§  Earthquake epicenters

§  Earth magnetism

§ Details of sea floor topography = bathymetry

Bathymetry

seamounts

abyssal plains

mid-ocean ridges

trenches

Screenshot from Google Earth

Earthquakes • More and better seismometers à more accurate

epicenters§  Patterns: EQs occur along ridges and trenches

https://earthquake.usgs.gov/earthquakes/search/

Earth Magnetism

1.  Magnetic rocks on land2.  Magnetic field strength over oceans

• Geophysicists studied

https://commons.wikimedia.org/wiki/File:Magnet0873.png

Iron filings outline magnetic field of a bar magnet.

Earth’s Magnetic Field

• Generated in the liquid outer core

§  From flowing metal

• Behaves like the field of a bar magnet

§ Varies with latitude (0° - 90°)

§  Angle of tilt = inclination

•  A freely-suspended magnet tilts:Detecting the Magnetic Field

Horizontal at equator

Vertical at poles

Angle of inclination increases as

latitude increases

Paleomagnetism •  Magnetic minerals in some rocks, mainly

igneous rocks, preserve magnetic field direction.

A Discovery using Paleomagnetism

1.  Polar Wander Paths

•  Method uses igneous rocks on land:

•  Discovery: MNP moves!

§  Age (from radioactivity)

§ Direction of magnetic north pole (MNP) (from paleomagnetism)

Polar Wander Paths for SA & AF

290 Ma

275 Ma

250 Ma 225 Ma 225 Ma

275 Ma

250 Ma

•  Implies 2 MNPs at the same time – impossible!https://www.researchgate.net/publication/253282658

Paths Coincide with Pangaea

275 Ma

250 Ma

225 Ma

https://www.researchgate.net/publication/253282658

Another Paleomagnetic Discovery

2. Magnetic reversals

•  Method: uses stacked lava flows on land

•  Discovery: field reverses!

§  Age (from radioactivity)

§  NMP (from paleomagnetism)

§  Compass points N

• Reverse polarity

§  Compass points S

• Normal polarity

Magnetic Polarity

One More Finding from Paleomagnetism

3. Marine magnetic anomalies

•  Method: ship tows magnetometer

• Discovery: High / low strength

§  Measures strength of field

§  Anomaly: variation from expected value

Terms for Anomalies

•  High strength = positive anomaly

•  Low strength = negative anomaly

•  Anomalies were a puzzle when first discovered!

Marine Magnetic Anomalies •  Alternating + and − anomalies

•  Symmetric across mid-ocean ridges

•  What causes these anomalies?

§  Positive over rocks with normal polarity§  Negative over rocks with reverse polarity

+−

Seafloor Spreading

•  Oceanic lithosphere:

§  Moves away from ridge

§  Sinks down into mantle at trenches & is destroyed

§  Forms at mid-ocean ridges

Provides a mechanism for continental drift!

• Harry Hess (1962)

§  Combined continental drift, bathymetry, EQs, magnetism

https://commons.wikimedia.org/wiki/File:Hess.gif

Explains symmetric anomalies!

New minerals align with magnetic field!

Patterns Match

Anomalies Reversals+ / − Normal/reverseWidth Duration

•  Seafloor spreading + reversals = anomalies

+−

Testing the SFS Hypothesis • Predictions: §  Youngest oceanic crust

at mid-ocean ridges§ Age increases with

distance from MOR

• Tested on hundreds of rocks: confirmed

• Rates of spreading = 1 – 17 cm/yr https://commons.wikimedia.org/wiki/File:Pacific_seafloor_crust_age_2.gif

Teaching Notes and Tips This exercise is divided into three complementary sections. The exercise may be completed in one extended laboratory period, or individual sections may be assigned as separate, shorter activities or as homework. The Excel workbook file includes a worksheet that contains the key. The workbook given to students should have only the Sediments worksheet. Note that students need access to a computer (Part III) and a printer (Part I) to complete this exercise. Alternatively, the instructor may print these materials as handouts. Some students have difficulty entering formulas in Excel, so the instructor should review the process and keep track of progress. Because computer software changes so rapidly, the instructions for accomplishing certain tasks with Excel might differ from those given in the student instructions. Thus, the instructor should be aware of possible difficulties using Excel.